1. Field of the Invention
The present invention generally relates to a pilot hydraulic system for operating a directional control valve adapted to control the operation of a hydraulic actuator. More specifically, the invention pertains to a pilot hydraulic system for operating a directional control valve which is suitable for incorporation in various working machines, such as hydraulic excavators, in which an inertial body acts upon the hydraulic actuator as a load.
2. Description of the Prior Art
In general, various types of working machines, such as hydraulic excavators, are equipped with appropriate hydraulic actuators. Such a working machine is commonly arranged to accomplish its predetermined motion by driving these hydraulic actuators in a suitable manner. Such hydraulic actuators are controlled by the directional control valves which ar driven through the operation of operating levers. In recent days, it has been proposed that the pilot operating system in which a directional control valve is driven by a pilot pressure. Generally speaking, such a conventional pilot operating system includes a pilot hydraulic system in which, as an example, a pair of pilot chambers are disposed at opposing ends of the directional control valve, with the pilot chambers being connected through pilot lines to a pilot valve operated by an operating lever.
The operation of such a prior-art pilot hydraulic system will be described below, with reference to a directional control valve for controlling the motion of a travel motor incorporated in the hydraulic excavator.
When an operator shifts the operating lever from its neutral position, a hydraulic fluid from a pilot hydraulic pump is made to flow through the pilot valve connected to the pilot hydraulic pump. This fluid is made to flow into one of the pilot lines, and is supplied to the pilot chamber of the directiohal control valve through pilot line. This inflow switches the directional control valve to a predetermined operating position which allows the flow of hydraulic fluid from a main hydraulic pump to the corresponding one of the two main lines through the directional control valve, so that the fluid is delivered to the travel motor connected to the main lines. After the fluid has completed its predetermined work within the travel motor, it is made to pass through the other of the main lines, returning to a reservoir through the directional control valve. The above-described sequence of hydraulic flow causes the rotation of the travel motor, and the hydraulic excavator is thereby made to travel.
When the operator returns the operating lever to the neutral position to stop the hydraulic excavator, the pilot valve first cuts off the communication between the pilot pump and the former of the pilot lines and this pilot line forms communication with the reservoir. The thus-formed communication allows the hydraulic fluid within the former pilot chamber of the directional control valve to flow back into the reservoir. Consequently, when the directional control valve is shifted to the neutral position, the supply of fluid from the main pump to the travel motor is interrupted and the main lines are closed. In the meantime, the travel motor does not immediately stop and will continue to rotate by inertial force. While the motor is rotating, it absorbs hydraulic fluid from the former main line and discharges the fluid to the latter main line. Therefore, the hydraulic pressure within the closed main line sharply increases, and the thus-increased pressure serves as brake pressure, thereby stopping the travel motor.
However, in such a prior-art hydraulic system, when the operating lever is returned to the neutral position, the directional control valve tends to restore to the neutral position remarkably quickly. Thus, the brake pressure generated within the latter main line shows a remarkably sharp rise, and also an extremely large impact is applied to the entire body of the hydraulic excavator when it is stopped. For this reason, the prior-art hydraulic system has generally involved various drawbacks, such as inferior operability of the system, a high level of operator fatigue, and deterioration of the operability of the mechanism. Such problems occur in various conventional types of hydraulic actuators used in the above-described hydraulic excavators and other working machines. In particular, as the load acting upon the hydraulic actuator shows a higher level of inertia, the influence of these problems over the entire mechanism becomes greater.